Water-insoluble, water-swellable, hydrogel-forming absorbent polymers are capable of absorbing large quantities of liquids such as water, body exudates or fluids (e.g., urine, blood, menstrual fluid), industrial fluids and household; fluids and are further capable of retaining such absorbed liquids under moderate pressures. The absorption characteristics of such polymer materials make them especially useful for incorporation into absorbent articles such as disposable diapers, adult incontinence pads and briefs, catamenial products such as sanitary napkins, and the like.
The development of highly absorbent members used in such absorbent articles are the subject of substantial commercial interest. A highly desired characteristic for such products is thinness. For example, thinner diapers are less bulky to wear, fit better under clothing, and are less noticeable. They are also more compact in the package, making the diapers easier for the consumer to carry and store. Compactness in packaging also results in reduced distribution costs for the manufacturer and distributor, including less shelf space required in the store per diaper unit.
The ability to provide thinner absorbent articles such as diapers has been contingent on the ability to develop relatively thin absorbent cores or structures that can acquire and store large quantities of discharged body fluids; in particular, urine. In this regard, the use of certain absorbent polymers often referred to as "hydrogels," "superabsorbents" or "hydrocolloid" material has been particularly important. See, for example, U.S. Pat. No. 3,699,103 (Harper et. al), issued Jun. 13, 1972, and U.S. Pat. No. 3,770,731 (Harmon), issued Jun. 20, 1972, which disclose the use of such absorbent polymers (hereafter "water-insoluble, absorbent, hydrogel-forming polymers") in absorbent articles. Indeed, the development of thinner diapers has been the direct consequence of thinner absorbent cores that take advantage of the ability of these hydrogel-forming absorbent polymers to absorb large quantities of discharged body fluids, typically when used in combination with a fibrous matrix. See, for example, U.S. Pat. No. 4,673,402 (Weisman et. al), issued Jun. 16, 1987 and U.S. Pat. No. 4,935,022 (Lash et. al), issued Jun. 19, 1990, which disclose dual-layer core structures comprising a fibrous matrix and hydrogel-forming absorbent polymers useful in fashioning thin, compact, non-bulky diapers.
Prior absorbent structures have generally comprised relatively low amounts (e.g., less than about 50% by weight) of water-insoluble, absorbent, hydrogel-forming polymers. There are several reasons for this. The hydrogel-forming absorbent polymers employed in prior absorbent structures have generally not had an absorption rate that would allow them to quickly absorb body fluids, especially in "gush" situations. This has necessitated the inclusion of fibers, typically wood pulp fibers, to serve as temporary reservoirs to hold the discharged fluids until absorbed by the hydrogel-forming absorbent polymer.
More importantly, many of the known hydrogel-forming absorbent polymers exhibited gel blocking when they are used in absorbent articles in a high concentration. "Gel blocking" occurs when particles of the hydrogel-forming absorbent polymer are wetted and the particles swell so as to inhibit fluid transmission to other regions of the absorbent structure. Wetting of these other regions of the absorbent member therefore takes place via a very slow diffusion process. In practical terms, this means acquisition of fluids by the absorbent structure is much slower than the rate at which fluids are discharged, especially in gush situations. Leakage from the absorbent article can take place well before the particles of hydrogel-forming absorbent polymer in the absorbent member are fully saturated or before the fluid can diffuse or wick past the "blocking" particles into the rest of the absorbent member. Gel blocking can be a particularly acute problem if the particles of hydrogel-forming absorbent polymer do not have adequate gel strength and deform or spread under stress once the particles swell with absorbed fluid. See U.S. Pat. No. 4,834,735 (Alemany et. al), issued May 30, 1989.
This gel blocking phenomena has typically necessitated the use of a fibrous matrix in which the particles of hydrogel-forming absorbent polymer are dispersed. This fibrous matrix keeps the particles of hydrogel-forming absorbent polymer separated from one another. This fibrous matrix also provides a capillary structure that allows fluid to reach the hydrogel-forming absorbent polymer located in regions remote from the initial fluid discharge point. See U.S. Pat. No. 4,834,735 (Alemany et. al), issued May 30, 1989. However, dispersing the hydrogel-forming absorbent polymer in a fibrous matrix at relatively low concentrations in order to minimize or avoid gel blocking may lower the overall fluid storage capacity of thinner absorbent structures. Using lower concentrations of these hydrogel-forming absorbent polymers limits somewhat the real advantage of these materials, namely their ability to absorb and retain large quantities of body fluids per given volume.
In general, increasing the gel strength of hydrogel-forming absorbent polymers can contribute to decreased gel blocking. Gel strength relates to the tendency of the hydrogel formed from these polymers to deform or "flow" under usage stresses. Gel strength needs to be such that the hydrogel formed does not deform and fill to an unacceptable degree the capillary void spaces in the absorbent structure or article, thereby inhibiting the absorbent capacity of the structure/article, as well as the fluid distribution through the structure/article. High gel strength is usually obtained by crosslinking. It is believed that crosslinking increases the resistance to deformation of hydrogel-forming absorbent polymer surfaces. However, normal crosslinking has a deep impact on the absorbent capacity of a hydrogel-forming absorbent polymer. In general, absorbent capacity or "gel volume" has inverse power-law dependence on the level of crosslinking. That is, high crosslinking level results in high gel strength but low gel volume. Gel volume be a measure of the amount of water or body fluids that a given amount of hydrogel-forming polymer can absorb. It is required that gel volume is sufficiently high in order that the hydrogel-forming polymer can absorb significant amounts of the aqueous body fluids encountered during use of the absorbent article.
Another important factor that has to be considered is the liquid permeability of hydrogel-forming absorbent polymers. It has been discovered that the permeability or flow conductivity of the gel layer formed by swelling in the presence of body fluids is extremely important when these absorbent polymers are used in absorbent cores or members at a high concentration in localized or throughout regions thereof. It should be noted that lack of liquid permeability or flow conductivity of absorbent polymers may directly impact on the ability of resultant gel layers to acquire and distribute body fluids.
Based on the foregoing, there is a need for an absorbent material which has improved fluid absorbency and fluid permeability.